Hydrolysis is a chemical reaction or process in which a molecule is split into two parts by reacting with a molecule of water, which has the chemical formulaH2O. One of the parts gets an OH- from the water molecule and the other part gets an H+ from the water.

This is distinct from a hydration reaction, in which water molecules are added to a substance, but no cleavage occurs. In organic chemistry, hydrolysis can be considered as the reverse or opposite of condensation, a reaction in which two molecular fragments are joined for each water molecule produced. As hydrolysis may be a reversible reaction, condensation and hydrolysis can take place at the same time, with the position of equilibrium determining the amount of each product. In inorganic chemistry, the word is often applied to solutions of salts and the reactions by which they are converted to new ionic species or to precipitates (oxides, hydroxides, or salts). Some examples of hydrolysis are explained below.

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Many metal ions are strong Lewis acids, and in water they may undergo hydrolysis to form basic salts. Such salts contain a hydroxyl group that is directly bound to the metal ion in place of a water ligand. For example, aluminium chloride undergoes extensive hydrolysis in water, such that the pH of the solution become quite acidic:

Hydrolysis of a hydrated Al3+ ion

This means that if solutions of AlCl3 are evaporated, hydrogen chloride is lost and the residue is a basic salt (in this case an oxychloride) in place of AlCl3. Such behaviour is also seen with other metal chlorides such as ZnCl2, SnCl2, FeCl3 and lanthanide halides such as DyCl3. With some compounds such as TiCl4, the hydrolysis may go to completion and form the pure hydroxide or oxide, in this case TiO2.

The fragment of the parent molecule that was originally a carboxylate gains a hydrogenion from the additional water molecule. The fragment that was originally an alkyl group collects the remaining hydroxyl group from the water molecule. This effectively reverses the esterification reaction, yielding the original alcohol and carboxylic acid again.

There are two main methods for hydrolysing esters, basic hydrolysis and acid-catalysed. With acid-catalysed hydrolysis a dilute acid is used to protonate the carbonyl group in order to activate it towards nucleophilic attack by a water molecule. However the more usual method for ester hydrolysis involves refluxing the ester with an aqueous base such as NaOH or KOH. Once the reaction is complete, the carboxylate salt is acidified to release the free carboxylic acid.

In other hydrolysis reactions, such as hydrolysis of an amide link into a carboxylic acid and an amine product or ammonia, only the carboxylic acid product has a hydroxyl group derived from the water. The amine product (or ammonia) gains the remaining hydrogen ion. A more specific case of the hydrolysis of an amide link is hydrolyzing the peptide links of amino acids.

Under physiological conditions (i.e. in dilute aqueous solution), a hydrolytic cleavage reaction, where the concentration of a metabolic precursor is low (on the order of 10-3 to 10-6 molar), is essentially thermodynamically irreversible. To give an example:

A + H2O → X + Y

Assuming that x is the final concentration of products, and that C is the initial concentration of A, and W = [H2O] = 55.5 molar, then x can be calculated with the equation:

let Kd×W = k:

then

For a value of C = 0.001 molar, and k = 1 molar, x/C > 0.999. Less than 0.1% of the original reactant would be present once the reaction is complete.

This theme of physiological irreversibility of hydrolysis is used consistently in metabolic pathways, since many biological processes are driven by the cleavage of anhydrouspyrophosphate bonds.